Weird line in my mirrored mesh
Problem

This is caused by faces inside your mesh. The main sign of this is that the crease only appears when subsurf is turned up; the inside faces add weight that pulls the verticies inwards (according to the subsurf algorithm).

Solution

Tab into edit mode and uncheck the X box in the Mirror modifier. Your properties panel should look like this:



This will leave you with half a human. Change into face select mode:  and select these faces:



Key X and select Faces. Tab out of edit mode and recheck X in the Mirror modifier. This is the repaired mesh: